OS X Mavericks' uptake slowed significantly in December, putting a crimp on Apple's plans to move customers to the new -- and free -- operating system.

Data from Web metrics company Net Applications also showed a worrisome fragmentation of the Mac operating system market, with three older editions, including 2009's Snow Leopard, crowded around the 20% line, each accounting for between one-sixth and one-fourth of the user base.

The slowdown in Mavericks adoption last month was in contrast to the rapid uptake in November, when the free operating system upgrade vaulted from fourth place among the various editions to first.

At the end of November, 32% of all Macs tracked online by Net Applications were running OS X Mavericks, a 21-point increase over the month prior. In December, Mavericks gained just 5 percentage points, climbing to a 37% share.

Still, Mavericks' December gain was twice that of OS X Mountain Lion in the second full month of its availability.

As happened the previous month, the bulk of Mavericks' December gain came at the expense of Mountain Lion, which fell 2.5 percentage points, and Lion, which dropped 1.4 points.

OS X Snow Leopard again resisted major defections, losing just seven-tenths of a percentage point in December to slip to 19.5% of all editions. Users of Snow Leopard have stubbornly stuck with their aged operating system -- which may have already seen its final security update -- in large part because it was the last version of OS X to let them run applications compiled for the PowerPC processor.

Mavericks' slowing adoption rate resulted in a cluster of editions around the 20% mark, with Mountain Lion accounting for 22%, Snow Leopard for 19.5% and Lion for 16.3% in December.

That fragmentation -- four editions with more than 16% of the Mac user share -- is exactly what Apple was hoping to solve by making Mavericks free.

"What's most important to us is seeing the software in the hands of as many Mac users as possible," said Craig Federighi, Apple's top software engineering executive, two months ago when he announced that Mavericks would be free.

The fragmentation issue will be long-lasting if past adoption trends are repeated. By Computerworld's calculations -- using six-month averages for each edition -- Mountain Lion will remain above 10% until April, Lion until May and Snow Leopard until January 2015.